1. Field of the Invention
The present invention is directed generally to communication systems and, more particularly, to a system and method for adaptive modulation.
2. Description of the Related Art
A wireless data system should ideally select a form of modulation and coding to make the most efficient use of the radio channel. The quality of the radio link between a transmitter and receiver determines which combinations of modulation and coding can be used for data.
Fixed lines may perform an extended channel response measurement at the outset to determine the most effective modulation. For example, digital subscriber line (DSL) technology uses bit loading for orthogonal frequency division multiplexing (OFDM) over phone lines. The channel response is measured at startup, and modulation is varied tone-by-tone to maximize the overall throughput. Each terminal must know the modulation used for each tone. Upon completion of the line testing, the particular form of modulation and coding are selected to maximize data transfer rates. Although this approach may be thorough, it is time consuming and relies on the channel being largely static and unchanging over time. As such, it is not appropriate for the wireless environment.
Unlike wired systems, the radio channel is highly dynamic. Those skilled in the art will recognize that a number of factors, both predictable and unpredictable, combine to affect a particular wireless link. For example, radio frequency interference, relative motion between the transmitter and receiver, and other factors decrease the effectiveness of long-term channel estimates that are used in wired systems.
Prior art techniques describe the use of special training symbols for time synchronization and channel estimation. Such systems require that a special training symbol precede an air link burst containing one or more OFDM symbols. Because this prior art technique relies on one or more pilot OFDM symbols being transmitted, it is less efficient for short bursts and in environments with multi-user contention for the airlink.
Therefore, it can be appreciated that there is a significant need for system and method for adapting modulation techniques to changing airlink conditions. The present invention provides this and other advantages as will be apparent from the following detailed description and accompanying figures.